A properly sized water heater meets your household's hot water demand efficiently. Before you buy, it's worth taking a few minutes to calculate the right size. This guide covers tankless, solar, and storage or heat-pump water heaters — including a simple worksheet to estimate your peak-hour demand.
Sizing Tankless or Demand-Type Water Heaters
Tankless water heaters are rated by two key metrics: maximum flow rate (gallons per minute) and temperature rise. To size one correctly, you need to know how much water your household draws at once and how cold your incoming water is.
Count Your Simultaneous Hot-Water Devices
List every fixture or appliance that could be running hot water at the same time — showers, faucets, dishwashers, and washing machines all count.
Add Up the Flow Rates
Each fixture has a flow rate measured in gallons per minute (GPM). A standard hot water faucet runs about 0.75 GPM; a shower head typically uses 2.5 GPM. Add these together for your required total flow rate.
Estimate the Flow Rate If You Don't Know It
Place a bucket or measuring cup under the faucet or shower head, run it for one minute, and measure what you collect. To lower your flow rate, install low-flow fixtures.
Calculate the Required Temperature Rise
Subtract your cold incoming water temperature from your desired output temperature. If you don't know your groundwater temperature, assume 50°F (10°C). Most uses call for 120°F (49°C) output — so in many areas you need a rise of about 70°F (39°C). For dishwashers without internal heaters, target 140°F (60°C).
Many tankless models are thermostatically controlled, meaning they can vary output temperature in response to the flow rate and inlet temperature — giving you consistent hot water across varying demand conditions.
Sizing a Solar Water Heating System
Solar water heating systems are sized to meet 90–100% of your household's hot water needs over the summer. Solar contractors typically use worksheets and computer modeling to determine the right collector area and storage volume for your location and usage pattern.
Collector Area
A standard guideline is about 20 square feet (2 sq. meters) of collector for the first two people in your household. Add approximately 8 square feet per additional person if you live in the U.S. Sun Belt, or 12–14 square feet if you're in the northern United States.
Storage Volume
A small 50–60 gallon tank suits one to three people. An 80-gallon tank works well for three to four. For active solar systems, allow roughly 1.5 gallons of storage per square foot of collector — and up to 2 gallons in very warm, sunny climates to prevent overheating.
Additional factors include your building's solar resource, the optimal panel orientation, and tilt angle. Consult a qualified solar contractor for a site-specific assessment before purchasing.
Sizing Storage & Heat Pump (Tank) Water Heaters
For storage-style water heaters — including heat pump models with a tank — the key metric is the first-hour rating (FHR). This number tells you how many gallons of hot water the heater can supply in one hour, starting from a full tank. It depends on tank capacity, the heat source (burner or element), and its size.
Look for the FHR in the top-left corner of the EnergyGuide label, listed as "Capacity (first-hour rating)." The Federal Trade Commission requires this label on all new conventional storage water heaters. To match the right heater to your household, find a model whose FHR at least equals your peak hour demand — the highest volume of hot water used during any single one-hour window in your day.
To estimate your peak hour demand, think about which time of day — morning, noon, or evening — your household uses the most hot water. Then use the worksheet below to add up your usage during that one-hour window.
| Use | Avg. Gallons per Usage | Times Used in 1 Hour | Gallons in 1 Hour |
|---|---|---|---|
| Shower | 20 | × | = |
| Shaving (0.05 gal/min) | 2 | × | = |
| Hand dishwashing or food prep (2 gal/min) | 3 | × | = |
| Automatic dishwasher | 7 | × | = |
| Clothes washer — Top-loader | 25 | × | = |
| Clothes washer — HE/Front-loader | 15 | × | = |
| Total Peak Hour Demand | = | ||
Example Household
In this example, the household needs a water heater with a first-hour rating of at least 66 gallons. Look for models that meet or exceed this figure.
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* Estimates are based on averages drawn from a range of published industry sources. Actual usage will vary by household size, fixture type, and usage habits. Some water heater manufacturers provide online calculators that account for specific use-case duration and frequency — check the manufacturer's website for additional guidance.